Re: [PATCH v3 00/30] qapi/qom: QAPIfy --object and object-add

Kevin Wolf <kwolf(a)redhat.com&gt; writes: > Am 13.03.2021 um 14:40 hat Markus Armbruster geschrieben: >> Markus Armbruster <armbru(a)redhat.com&gt; writes: >> >> > Paolo Bonzini <pbonzini(a)redhat.com&gt; writes: >> > >> >> On 11/03/21 15:08, Markus Armbruster wrote: >> >>>> I would rather keep the OptsVisitor here. Do the same check for JSON >> >>>> syntax that you have in qobject_input_visitor_new_str, and whenever >> >>>> you need to walk all -object arguments, use something like this: >> >>>> >> >>>> typedef struct ObjectArgument { >> >>>> const char *id; >> >>>> QDict *json; /* or NULL for QemuOpts */ >> >>>> QSIMPLEQ_ENTRY(ObjectArgument) next; >> >>>> } >> >>>> >> >>>> I already had patches in my queue to store -object in a GSList of >> >>>> dictionaries, changing it to use the above is easy enough. >> >>> >> >>> I think I'd prefer following -display's precedence. See my reply to >> >>> Kevin for details. >> >> >> >> Yeah, I got independently to the same conclusion and posted patches >> >> for that. I was scared that visit_type_ObjectOptions was too much for >> >> OptsVisitor but it seems to work... >> > >> > We have reason to be scared. I'll try to cover this in my review. >> >> The opts visitor has serious limitations. From its header: >> >> * The Opts input visitor does not implement support for visiting QAPI >> * alternates, numbers (other than integers), null, or arbitrary >> * QTypes. It also requires a non-null list argument to >> * visit_start_list(). >> >> This is retro-documentation for hairy code. I don't trust it. Commit >> eb7ee2cbeb "qapi: introduce OptsVisitor" hints at additional >> restrictions: >> >> The type tree in the schema, corresponding to an option with a >> discriminator, must have the following structure: >> >> struct >> scalar member for non-discriminated optarg 1 [*] >> list for repeating non-discriminated optarg 2 [*] >> wrapper struct >> single scalar member >> union >> struct for discriminator case 1 >> scalar member for optarg 3 [*] >> list for repeating optarg 4 [*] >> wrapper struct >> single scalar member >> scalar member for optarg 5 [*] >> struct for discriminator case 2 >> ... > > Is this a long-winded way of saying that it has to be flat, except that > it allows lists, i.e. there must be no nested objects on the "wire"? I think so. > The difference between structs and unions, and different branches inside > the union isn't visible for the visitor anyway. Yes, only the code using the visitor deals with that. >> The "type" optarg name is fixed for the discriminator role. Its schema >> representation is "union of structures", and each discriminator value must >> correspond to a member name in the union. >> >> If the option takes no "type" descriminator, then the type subtree rooted >> at the union must be absent from the schema (including the union itself). >> >> Optarg values can be of scalar types str / bool / integers / size. >> >> Unsupported visits are treated as programming error. Which is a nice >> way to say "they crash". > > The OptsVisitor never seems to crash explicitly by calling something > like abort(). > > It may crash because of missing callbacks that are called without a NULL > check, like v->type_null. Correct. > This case should probably be fixed in > qapi/qapi-visit-core.c to do the check and simply return an error. I retro-documented what I inherited: qapi-visit-core.c code expects the visitors to implement the full visitor-impl.h interface, but some visitors don't. So I documented "method must be set to visit FOOs" in visitor-impl.h, and for the visitors that don't, I documented "can't visit FOOs". If the crashing behavior we've always had gets in the way, there are two ways to change it: 1. Complicate qapi-visit-core.c slightly to cope with incomplete visitor implementations. 2. Complete the visitor implementations: add dummy callbacks that fail. I prefer 2., because I feel it keeps the visitor-impl.h interface simpler, and puts the extra complications where they belong.